The present invention relates to improvements in and/or relating to multi-layer heat shrinkable plastic films, containers (for example bags ) or packages using same and to related means and methods.
Multi-layer heat shrinkable films have been used for some time for the shrink wrapping of various products including produce such as meats that will require refrigeration or freezing. Desirable characteristics in such films is their degree of shrinkage under moderate heat and their resistance to abuse (usually just penetration by the product).
Currently available in New Zealand are at least two films useful in the wrapping of produce such as meats. These include the Ionomer based materials employed in the SURLYN bags of Trigon Packaging Systems (NZ) Limited of Corner Avalon Drive and Foreman Road, Hamilton, New Zealand and the EVA (ethylene-vinyl acetate copolymer) based materials of W. R. Grace (NZ) Limited of Prosser Street, Elsdon, Porirua which are known as their SL3 and SB3 bags.
The abuse resistance of the films if improved would make such containers more useful in the packaging of produce having a greater tendency to penetrate or puncture the packaging. For example there is a high incidence of puncture where a bone is retained in a meat cut.
Also, it is essential for such films to be "high shrink" as these are important from both presentation appeal and consumer appeal points of view. A typical example of a "high shrink" EVA structure is the SL3 film of W. R. Grace (NZ) Limited with a minimum of 40 to 50% shrinkage when unrestrained.
The present invention therefore is directed to films, container, packages and methods of producing such films for example (by co-extrusion or coating extrusion) which will go at least someway to meet the above mentioned aim.
As used hereinafter the abbreviations EVA refers to an ethylene-vinyl acetate copolymer, VLDPE to very low density polyethylene polymer (preferably less than 0.915 gms/cc), PVdC to vinylidene chloride/vinyl chloride copolymer, and MA to vinylidene chloride methyl acrylate copolymer, used as a barrier layer to reduce the permeation of oxygen and other gases through the film thus reducing the deterioration of products contained in the film.
In a first aspect the present invention consists in a method of producing a multi-layer heat shrinkable film having a measure of abuse-resistance greater than that of EVA (ethylene-vinyl acetate copolymer) of a heat shrinkable quality and a shrinkability greater than that of VLDPE (very low density polyethylene polymer), or a precursor for such a film, comprising
co-extruding as a multi-layer film precursor laminate, at least one layer of a high EVA content and at least one other layer each selected from the group comprising VLDPE, a blend substantially of VLDPE and a blend of VLDPE and EVA to provide the precursor, and
subsequently treating the precursor to provide the heat shrinkable film.
Preferably said laminate is of two layers only.
Preferably said two layer combinations are selected from the group comprising VLDPE/EVA layers and VLDPE+EVA/EVA layers.
Preferably said EVA of said at least one layer of high EVA content has about a 19% by weight vinyl acetate (VA) content.
Preferably said EVA of said at least one other layer has about 12% by 10 weight vinyl acetate (VA) content.
Preferably said precursor laminate is irradiated to enhance its shrink and strength characteristics.
Preferably said irradiation is with a high energy electrons giving an absorbed irradiation dose of from 1 to 9 MegaRads.
Preferably said irradiation is with high energy electromagnetic radiation giving an absorbed dose of from 1 to 9 MegaRads.
Preferably the precursor laminate is then stretched by racking and/or blowing to a desired film thickness to thereby provide the heat shrinkable film.
Preferably the stretching is from 2 to 30 times the surface area of the film precursor.
Preferably the stretching is to about 10 times the surface area of the film precursor.
Preferably said method additionally comprises co-extruding onto the irradiated laminate at least one layer each selected from the group comprising EVA, VLDPE, and a blend thereof.
Preferably said one or more layers co-extruded onto the irradiated laminate is a blend of EVA and VLDPE.
Preferably said method comprises co-extruding onto the irradiated laminate at least one layer of VLDPE.
Preferably a barrier film to provide a better barrier to oxygen and other gases is co-extruded in conjunction with the at least one other layer being applied to the irradiated laminate.
Preferably said barrier layer is interposed between the irradiated laminate and at least one other layer co-extruded onto the irradiated laminate.
Preferably the precursor laminate for the multi-layer heat shrinkable film has a thickness of from 100 to 1000 microns.
Preferably the thickness is about 800 microns.
Preferably said precursor laminate produced by a method of the present invention previously defined is then stretched by racking and/or blowing to a desired film thickness to provide the heat shrinkable film.
Preferably said desired film thickness is in the range from about 50 to 150 microns.
Preferably the film thickness is in the range from 50 to 100 microns,
Preferably said film thickness is about 80 microns.
In a further aspect the present invention consists in a multi-layer heat shrinkable film having a measure of abuse-resistance greater than that of EVA of a heat shrinkable quality and a shrinkability and clarity greater than that of VLDPE when made by a method as previously defined or stretched from a precursor as previously defined.
Accordingly in another aspect the present invention consists in a multi-layer heat shrinkable film having good resistance to abuse, said film comprising at least three layers, one outside layer being VLDPE or a blend of EVA and VLDPE (hereinafter the "outside layer"), the other outside layer being VLDPE or a blend of VLDPE and EVA (hereinafter the "inside layer"), and an or the internal layer being EVA or primarily EVA.
Preferably the or an internal layer is EVA alone.
Preferably there is included at least one barrier layer.
Preferably said barrier layer is PVdC or MA.
Preferably said outside layer is a blend in the range of from 10 to 75% by weight EVA and from 90 to 25% by weight VLDPE.
Preferably the blend is in the range from 25 to 75% by weight EVA and from 75 to 25% by weight VLDPE.
In another preferred form the outside layer is VLDPE alone.
Preferably the inside layer is VLDPE alone. In other preferred formed however the inside layer is a blend of VLDPE and EVA.
Preferably the relative thicknesses of the layers are such that (ignoring any barrier layer) the inside layer is the thinnest.
Preferably there are four layers where the optional barrier layer is included.
Preferably said VLDPE has a density below 0.915 gms/cc.
Preferably said VLDPE has a density of between 0.912 and 0.905 gms/cc.
In a still further aspect the present invention consists in a multi-layer heat shrinkable film having good resistance to abuse, said film comprising at least three layers, there being an outside layer of a blend of from 10% to 75% by weight EVA and 90% to 25% by weight VLDPE, an inside layer (i.e. produce contacting) of VLDPE or a blend of VLDPE and EVA, at least one inside layer of EVA or primarily of EVA and an optional barrier layer (preferably of PVdC or MA).
Preferably the film as substantially as hereinafter described in detail.
In still a further aspect the invention consists in a container or packaging (or a package) wherein at least part thereof is a multi-layer heat shrinkable film in accordance with the present invention and wherein the outside layer (i.e. that layer not to contact the contents of any such container or package) being the blend of from 10 to 75% by weight EVA and 90 to 25% by weight VLDPE.
In yet a further aspect the invention consists a method of manufacture by extrusion which results in a multi-layer heat shrinkable film in accordance with the present invention.
In still a further aspect the present invention consists in a multi-layer heat shrinkable film having good resistance to abuse, said film stretched from a film precursor laminate comprising at lest three layers, there being
(i) an outside layer of a blend of from 10% to 75% by weight EVA and 90% to 25% by weight VLDPE,
(ii) an inside layer (i.e. produce contacting) of VLDPE or a blend of VLDPE and EVA, and
(iii) at least one layer of EVA or primarily of EVA and an optional barrier layer sandwiched by said inside and outside layers.
Preferably said barrier layer is present and is of PVdC or MA.
Preferably said inside layer and a contiguous EVA layer forming part of the sandwiched layers (iii) has been irradiated while the remaining layers have not.